Power fluctuation protection apparatus



Oct. 26, 1965 c. w. M|| l s POWER FLUCTUATION PROTECTION APPARATUS 2Sheets-Sheet 1 Original Filed Aug. 28. 1961 O. mUIDGw Oct. 26, 1965 c.w. MILLS POWER FLUCTUATION PROTECTION APPARATUS Original Filed Aug. 28,

2 Sheets-Shed'I 2 l Qm Non lwm Alp

3,214,640 Patented Oct. 26, v1965 3,214,640 POWER FLUCTUATION PROTECTIONAPPARATUS Charles rW. Mills, Greenville, S.C., assignor to W. R. Grcet8; Co., Duncan, S.C., a corporation of Connec lcu Continuation ofapplication Ser. No. 134,238,

Au 28 1961. g

This application Jan. 14, 1965, Ser. No. 427,539`

6 Claims. (Cl. 317-31) v This is a continuation of a copendingapplication, Serial No. 134,238, iiled August 28, 1961, now abandoned.

This invention relates to apparatus for protecting electrical equipmentfrom sudden transient dips in the line voltage supplied from analternating current source. More particularly, the invention relates tomeans for monitoring the supplyline voltage and providing a controlsignal for disconnecting the equipment from the line when a power dip orother short term transient fluctuation occurs inthe supply voltage.

Certain types 4of electrical equipment in use today are highlysusceptible to damage when a momentary drop in line voltage occurs whichis then followed by an immediate reapplication of full line potential.Under such conditions there is great likelihood that conventionalholding circuits,circuit breakers, and the like will notbe responsiveto, and hence not be activated by, such transient fluctuations in theline voltage, lastingfor a fraction of a cycle or even a few cycles atmost. When the voltage is` restored, the sudden application of full linepotential will then frequently result in severe strain on theelectromechanical elements of an electrical system such asmotorgeneratorsets, for example, and, in addition, will cause violentand oftentimes destructive current surges `in inductive elements of thesystem such as transformers and the like. Therefore, the need exists fora sensitive protective device which is capable of monitoring the A.C.line potential, with a suiciently fastresponse time to detect transientvoltage fluctuations lasting for extremely short'inte-rvals of time, andproviding a control signal for activating suitable drop-out circuitryfor disconnecting the electrical system from the power line when suchV apotentially harmful condition exists.

Prior art devices for protecting electrical equipment in the event ofpower line failure, by removal of the equipment `from the line,generally suiferfrom the disadvantage that the response time requiredfor their activation'is usually so long that they are unable to reactfast enough, and thereby provide the necessary protection;

for the system, when a short-term voltage fluctuation, lasting a fewcycles at the most, occurs. The present invention overcomes this majordisadvantage of conventional protection devices and provides novelelectronic means for detecting and responding to transient power linevoltage fluctuations. Y

A feature of the proposed apparatus is the-provision of a gas-type lamp,connected to the line, which is adapted to respond very rapidly to linevoltage, and a photosensitive device for sensing radiation from thelamp, whereby in normal operation the lamp is illuminated andextinguished at double the line frequency, and the photosensitive deviceproduces a train of double-frequency pulses of Vpredetermined Waveform.A waveform-sensing circuit connected to the photosensitive device, and acontrol circuit controlled thereby, is adapted to respond monitored bythe protective device. The radiation emanating from this lamp impingesupon a photocell whose output current is amplied and applied to adetector which recties and transmits the A.C. component of the photocellsignal waveform. The output of this Adetector is then applied to thecontrol grid of. a.ga's iilledl thyratron tube through "an RC(resistance-capacitance) circuit having a variable time constant. Thistime con-v stant is adjusted such that under normal operating condi-Itions, that is, when the A.C. line potential being monitored isuninterrupted by power fluctuations, the capacitor of the RC combinationis re-charged to a sufficiently negative potential on each cycle oftherectied photocell signal waveform that thecontrol grid of thethyratron is continuously maintained at a negative bias potential withrespect to its cathode, thus preventing its ring. However, when amomentary dip in the line potential takes place, the signal derived fromthe lamp and photocell combination fluctuates accordingly, and causesthe current supplied by the detector to be insufficient to prevent thecapacitor from discharging towardsV zero potential, and the control gridthen goes sufficiently in a positive direction Vso as to cause thethyratron to fire. When this situation occurs and the thyratron fires,the resultant plate` c.p.s frequency may be detected by the device andcause it to respond. Through variation of the time constant of theARCvcombination, the sensitivity of the protection device may be adjusted tohave a threshold response, i.e., the thyratron willre and therebygenerate a control signalonly when the momentary power losslasts forIapre-"-y determined length of time. yThis feature 4is particularly-fadvantageous when itis known that only power interruptions lasting for agiven timeduration will cause deleterious effects on a particularelectrical system, and that the effects of power 4interruptions ofshorter duration will' bei' negligible.

The foregoing and other objects, features and advantages of the presentinvention will bemore readily understood from a consideration of thefollowing detailed description of a preferred embodiment of theinvention,

4as illustrated in the accompanying drawings: i FIGURE 1 isa blockdiagram of a preferred embodiment of the invention showing the controlsignalgenerated by the protective apparatus activating a Adrop-outcircuit whichV precedes thefelectrical system which lis to beprotectedfrom transient voltagefluctuations;-

FIGURE 2 isan electrical schematic of a suitable circuit rneans-forpracticing the invention in accordance with the embodiment set forth inFIGURE 1.

rFIGURE 3 is a series of wave forms having a common time basis whichwill be helpful in understanding the4 theory and operation of theembodiment of the invention shown in FIGURES 1 and 2.

vFIGURE 4 is a curve showing the discharge characteristics of aresistance-capacitance circuit combination.

FIGURE 5 is a curve illustrating the changes accomplished in thedischarge rate of va resistance-capacitance combination throughvariation of its time constant.

Referring now to FIGURE 1, there is shown a source of alternatingcurrent potential 10, supplying electrical power to an electrical system20 preceded by a drop-out circuit 15. The drop-out circuit may be ofconventional design, comprising circuit breakers or various types ofcurrent interrupting devices, whereupon the receipt of an electricalcontrol signal activates a suitable mechanism for disconnecting theelectrical system 20 from the power source 10. The type of drop-outcircuit is generally determined by the nature and size of the electricalsystem which is to be protected; however, magnetic controllers, which intheir normal state are closed and which open upon receipt of anelectrical control signal, may be satisfactorily utilized as circuitbreakers in a wide range of electrical systems of the type previouslydescribed.

The power line voltage is monitored by a protection apparatus 30 whichcomprises lamp 32, photosensitive element 34, amplifier 36, detector 38,time constant circuit 40, and control element 42. The lamp 32 which isdirectly connected to the A.C. power source has a fast responsecharacteristic; that is, the intensity of its illumination rapidlyfollows the instantaneous magnitude of the energizing potential appliedacross its terminals. One class of lamps having this characteristic andfound suitable for the present application is the gaseous-glowdischargetype. Such lamps generally have a pair of electrode elements which areenclosed in a sealed tube lled at low pressure with .an inert gas, suchas neon, argon, and the like. Depending upon the pressure of the gascontained in the tube envelope, a glow discharge will take place, i.e.,the lamp will light, when a voltage applied across the electrodesreaches a predetermined magnitude. The intensity of the glow dischargewill then vary as a function of the magnitude of the applied voltage solong as the voltage exceeds a certain minimum level. When the appliedvoltage recedes below this threshold level, the glow discharge will nolonger be capable of being maintained, and the lamp will beextinguished. The voltage at which the lamp is extinguished (thethreshold level for extinction) may or may not be the same as thethreshold voltage required for the glow discharge to be established vandthe lamp thereby lit. For purposes of simplifying the subsequentexplanation of the operation of the invention, it will be assumed,however, that these respective points are the same.

A thermally incandescent lamp, i.e., the ordinary electric light bulb,is not particularly suitable in this application, as its responsecharacteristic is relatively slow. Such a lamp, wherein the illuminationis generated from a heated filament, has a high thermal inertia suchthat, for the conventional 60 c.p.s. alternating power supply, thefilament does not cool down sufficiently on the descending portion ofthe voltage cycle for the light output of the lamp to approach anywherenear extinction. Furthermore, because of thermal inertia, this type oflamp will continue to radiate light for a time period equivalent to alarge number of cycles of the power supply after it is deenergized, asthe temperature of the filament slowly cools down. Accordingly, a lamphaving a fast response characteristic, such as the gas-filled dischargetube described, or in some applications, a fluorescent type lamp ispreferably employed in principal embodiments of the invention wherein itis desired to have a protective apparatus possessing the capability ofdetecting and responding to transient volt-age fluctuations lasting asbriefly as a fraction to a few cycles of the power supply waveform.

As the lamp 32 responds to the absolute magnitude of the voltagesupplied from the power source of alternating potential 10, it will beilluminated and extinguished at a rate which is double the frequency ofthe energizing power supply, and thus the lamp will be turned on and olftwice in each full cycle of the supply Voltage waveform.

The light radiation emanating from lamp 32 impinges upon aphotosensitive element 34, which may be exemplilied by a photocellgenerating a plate current component'or output in response to theintensity of illumination incident upon the cell. The output from thephotosensitive element 34 is in the nature of a periodic pulsatingwaveform having a frequency twice that of the alternating potentialsupplied by the power source 10. After removal of the average or D C.level in the photosensors output to -eliminate the effects of backgroundillumination, the pulsating output of the photosensitive element 34 isthen magnified by amplier 36 and applied to a detector or rectifierelement 38 which passes only that portion 0f the A C. component of theoutput or response signal having a particular polarity, for example, thenegative part of the waveform. The output of the detector 38, whichcomprises only the particular portion of the pulsating response signalwaveform having negative polarity, is then applied to the input of atwo-state control element 42, typically a thyratron, through a variabletime const-ant circuit 40. The time constant circuit 40 serves to smoothout the pulsating signal applied to the control element from thedetector 38 such that during normal, uninterrupted operation of thepower source 10 the voltage level at the input to the control element 42is biased above a predetermined threshold value so that the controlelement is continuously maintained in a irst state of operation.

When a power dip or lluctuati-on lof predetermined duration, asdetermined by tlhe setting of the parameters comprising the variabletime constant circuit 40 occurs,

the volta-ge .level at the input of the control element 42 decreases t-othe threshold bias value, thereby allowing the contr-ol element tochange states. Upon the shifting of the control element 42 from the rrstto the second state of 'operati-on, an electrical signal is .generatedat its output which is utilized to activate :a drop-out circuit d'5 'ofconventional design. 'Upon receipt of the control signal from thecontrol element 4'2, indicati-ng that a voltage interruption in thepower line has occurned, the drop-out circuit ltl'5 then disconnectsfrom the line the electrical equipment '20 which is of such nature thatit would be subject t-o severe damage if permitted to remain linked tothe A.C. power source 10. As one example of the type of electricalequipment 'wihere the fast activation of a line drop-out upon theoccurrence of a transient power dip is not only desirable butpractically essential t-o nondetrimental operation, the invention hereinproposed has been found to 'be especially suitable for use inconjunction with an electron beam generator system yby providingprotection both yagainst damaging current surges in a resonanttransformer :and destructive physical lstresses in a motor-generator setwhich together comprise .a major portion ofthe power supply ySor theelectron beam generatol".

FIGURE l2 is a schematic of a suitable electrical circuit means foraccomplishing the component functions indi-- cated in t-he respectiveblocks o'f tfhe diagrammatical embodiment of the invention shown inFIGURE 1. The individual circuit elements in FIG. 2 which .are exemplaryof, and correspond to, the particular block elements of FIGURE 1 areshown within d-otted block outlines fhaving the same reference numerals.In understanding the theory and yoperation of 'the embodiment of theinvention shown in 'FIGU'RE l2, it will Ibe lielpfful to make referenceto the series of curves shown in FIGURE 3, which illustrate, on a commontime basis, the waveforms appearing at various locations in the circuitdiagram.

As before, there is shown in FIG. 2 a source of alternating potential'10 connected by :a power line to electrical equipment 20 and having anoutput which is to be monitored for transient voltage fluctuations orpower Idips by the apparatus of the present invention. The sinusoidalwaveform of the alternating potential supplied `by the power source 10is illustrated in lFIG. 3A. rIlhis waveform has :a period indicated as2T, which, for the typical power line install-ation, would be equivalentto 1A@ of a second. A power il-uctuation or voltage dip, for purposes ofillustrating Ithe operating principles of the protective apparatus ofthe present invention, is shown as occur-ring exemplari'ly at 70 duringthe negative half of the se'cond cycle of the power supply waveorm.

A gas-filled tube of the glow discharge type is placed lacross `thepower line and energized by the alternating potential of the powersource 10. When the absolute value of the volt-age applied across thistube .approaches the level indicated in FIG. A3A by the dottedhorizontal lines 71a an'd 7|1b (which correspond to respective positive'and negative potentials), a glow discharge wil-l be established betweenthe elect-rode elements within the tulbe with the resultant emission oflight therefrom. As the rising voltage then exceeds this threshold levelof illumination shown .as 72, the intensity of the radiation generatedby the gaseous discharge will increase as a function of the increasing'voltage until the voltage reaches maximum point 76, .at which time theintensity of the light radiation will likewise reach .a maxi-mum. As thevoltage proceeds -to fall from this maximum point, the intensity of theradiation from the lamp 32a likewise decreases until the extinctionlevel "78 is reached wherein the absolute magnitude ot the voltage isinsuicient to main- Itain the gaseous discharge. For purposes otconvenience, the threshold level olf illumination 72 .and thresholdlevel of extinction 78 for the lamp 32a are shown to occur at thesame-voltage magnitude, indicated by the dotted lines 71a and 71h; however,it is to be expressly understood that the satisfactory -openation of theapparatus is in no` wise dependent upon this similarity. One type oflamp commercially available .and fonmd suitable for use in the presentinvention is the AR-l argon lamp manufactured bly Genenal Electric whichhas the characteristics described a' ove.

rDhe relative intensity of the light odtput of the lamp 82a .isillustrated in FIG. 3B. It will be observed that the Waveform of theintensity of the radiation emanating trom the lamp is of .a pulsatingnatur-e wherein the pipe |80 correspond to the regular, clipped-olf peakportion of the input voltage Waveform. Furthermore, the frequency ofthis `pulsating wave is twice that of the waveform of the input supplyvoltage -since light .intensity has no po- Ilarity, an'd therefore theoutput of the lamp on the negative thalf of the supply voltage cycle isidentical to that on the positive half of the cycle. The 'waveform ofthe inten` sity of thelamp radiation t-hus has a period T which isone-halt that of the input voltage waveform, `or 1/120 of a second. v

The intensity of the illumination emitted from t-he lamp 32a reaches .amaximum .point 82 Iat the same instantin' time that the supply voltagewaveform goes through a negatwe or positive maximum. Notice that,duringthe voltage half cycle in fwhich the power dip 70 occurs, only thoseportions, indicated as 83 and '84 in the waveform shown in FIG. 3A,Where the voltage exceeds the illuminat1on .and extinction levels of thelamp (72 and 78) are reflected as corresponding radiation outputs `fromthe lamp 32a in FIG. 3B.

h Illumination produced from the activation of the glow dlscharge in gastube 32a is incident upon a photocell 34a which is adapted'to have peakresponse to the wavelength of light radiated by the lamp. When the glowlamp is dark, the output or plate current of the photocell will beapproximatey zero. Accordingly, when plate current is not flowing, thepotential at the plate electrode of ,the

photocell will be at a maximum. Correspondingly, when plate currentflows as a result of light radiation from the illuminated glow lampimpinging upon the photocell, the plate potential will vary inaninversemanner; that is, the plate voltage will decrease as the plate currentincreases, with the voltage reaching a minimum when the light intensityis at a maximum, and vice versa. A curve of the potential appearing atthe plate electrode of the photocell 34a due to the incidence ofradiation from the glow tube 32a is shown in FIG. 3C, with thehorizontal dotted lines 90 andv 92 representing the respective maxi- 6mum and minimum plate voltage levels during normal operation of thealternating current power supply 10.

The voltage appearing at the plate of the photocell 34a is then coupledthrough a capacitor, which blocks the passage of the D.C. component ofthe photocell output, to a two-stage amplitier 36 of conventional designhaving a gain control 37. This amplifier may typically comprise a singletube having two separate triode sections enclosed therein saidassociated circuitry, one suitable tube for an application of this typebeing a 12AT7. The amplifier 36 is preferably provided with a highfrequency by-pass lter, as exemplied by capacitor 35 inserted betweenits two stages, for shunting to ground any transients of a spuriousnature which might be introduced from extraneous sources. As two stagesof amplification are provided by the amplifier 36, the pulsatingvoltage, representing the amplified A.C. component or response signal ofthe photocell 34a, appears at the output of the amplifier in phase withthe input signal taken from the plate of the photocell. The waveform ofthis amplified response signal is illustrated in FIG. 3D. With the D.C.component of the photocell output removed, the pulsating signal at theoutput of the amplifier 36 exhibits a waveform having a maximum positivevalue lying along the horizontal dotted line and a maximum negativevalue along the line 102, the average value of the waveform being zero.

This amplified response signal is then applied to a rectifier ordetector element, represented by the non-linear diode 38a., whichtransmits only the negative portion of the waveform, the positivecomponent being effectively blocked by the high reverse-directionimpedance of the rectifier element. appearing at the output terminal ofthe diode element 38a as it would appear if there were no energy storagecircuitry immediately following. This curve will be helpful inunderstand the nature of the actual voltage waveform applied to theinput of the time constant circuit 40, which illustratively comprises aparallel combination of a variable resistance element R and a capacitorC.

Since only the negative component of the signal waveform generated bythe photocell is transmitted by the detector 38d, there is a substantialportion of each cycle of the waveform when the instantaneous voltage iszero. This portion of the signal waveform cycle corresponds to thesummation of the voltage dead zone of the glow tube 32a, wherein theenergizing potential is insufficient to maintain the gaseous discharge,and that fraction of the waveform cycle occupied by the positivecomponent `of the protocell response signal which has been blocked bythe detector element 38a. It is to be noted that the smaller pip 84,which was present in the fourth cycle of the output signal from thephotocell as shown in FIG. 3B, is not represented at the output of thedetector as shown in FIG. 3E, since it did not possess any negativecomponent counterpart in the voltage signal appearing at the outp-ut ofthe amplifier 36 and applied to the input of the diode element 38a. Itwill be observed that, due to the voltage fluctuation or power loss 70occurring in the negative portion of the second cycle of the A C. supplywaveform (FIG. 3A), there exists a consi-derable period of time, Ti,between the third and fifth cycles of the pulsating signal waveform whenthe instantaneous voltage appearing at the output of the detectorelement 38a is zero, broken only by a very small negative voltage pip112 occurring during the fourth cycle of the waveform (FIG. 3E).

For purposes of illustration, the regular waveform of the signalappearing at the output of the detector, as shown in FIG. 3E, .is takento have a maximum negative value of' -4 volts, as represented by thedotted horizontal line 110. This value lof the negative maximum may bereadily altered through adjustment of the gain control 37 of thetwo-stage amplifier 36.

As previously stated, the output of the detector 38a is FIG. 3E showsthe voltage waveform` applied to a time constant circuit 40 comprisingthe parallel combination Iof variable resistor R and capacitor C.` Whenan alternating potential is applied across this combination, thecapacitor C, being an energy storing device, is charged up to a peakvalue E during the increasing or ascending portion of the voltage cycleand is discharged during the decreasing or descending portion of thecycle. The rate at which the capacitor is discharged, and hence theminimum voltage level to which it descends during a full cycle of theinput voltage waveform, is determined by the time constant of thecircuit 40. For the parallel combination of the resistor R and thecapacitor C shown, the time constant is mathematically equivalent to theproduct of the resistance of R in ohms and the capacitance of C infarads. The voltage e existing at any instant of time across thecapacitor C during the discharging portion of the waveform cycle is anexponential function and may be expressed bythe following formula:

e E e R C,

where E represents the initial or peak voltage across the capacitor atthe instant discharging commences, e is the base for natural logarithms,t is the time in seconds measured from the start of the dischargeperiod, and RC is the time constant for the circuit.

This eifect is exemplified in FIG. 4 wherein it is shown that, startingfrom an initial condition or peak value for E of -4 volts, the voltageacross the capacitor C steadily decays towards zero potential in anexponential manner according to the above formula. Thus, after a timeperiod has elapsed equal to the time constant RC, the voltage will havedecayed to 36.8% of its initial potential or -l.5 volts. Similarly,after a time period has elapsed equivalent to twice the time constant(ZRC), the voltage will fall to 13.5% of the initial value or 0.54 volt.The voltage across the capacitor will continue to decrease towards zeropotential until the voltage applied to the input of the time constantcircuit by the detector 38a ceases to be less than the instantaneouscapacitor voltage e and begins to rise above it on the ascending portionof the signal waveform. At the instant this occurs, the capacitor Cceases to discharge through the resistance R and begins instead to berecharged back to the peak value E while following the ascending portionof the signal waveform. For the particular conditions illustrated inFIG. 4, the charging of the capacitor C by the ascending portion of thesignal waveform iirst commences when the capacitor has discharged to 75%of its initial peak value or -3 volts, as shown at 126. Recharging bythe ascending portion of the signal Waveform continues until thecapacitor voltage e reaches its initial peak value of -4 volts,whereupon the cycle commences again. Thus, during regular, uninterruptedoperation of the power supply 10, the voltage appearing across thecapacitor C of the time constant circuit 40 will fluctuate between anegative maximum of -4 volts and a minimum of -3 volts.

The uctuating negative voltage appearing across the capacitor C of thetime constant circuit 40 is applied to the control grid 43 of athyratron tube 45, which is exemplary of the two-state control element42. During normal uninterrupted operation of the power supply thethyratron tube is maintained in the unred or oil state by the negativebias potential applied to its control grid 43 from the output of thetime constant circuit 40. As long as the control grid is maintainedsufficiently negative with respect to the cathode 46 of the thyratron,current will not ow in the circuit of the plate electrode 44. If,however, the control grid voltage should be permitted to decrease in apositive direction to a critical threshold value, the grid will nolonger be capable of preventing electrons drawn from the cathode 46 fromflowing to the plate electrode 44, and the thyratron will re. Thegas-filled electron discharge tube, which the thyratron typiiies, hasthe important characteristic that ring, ie.,

8.V the initiation of plate current ow, takes place extremely rapidlyonce the control grid potential drops to the critical threshold value.Accordingly, the thyratron tube may be expeditiously utilized as acontrol element for deriving an output signal from the plate circuitwhen an input signal of proper magnitude and polarity is applied to itscontrol grid thereby causing the element to change states'.

In the embodiment of the invention illustrated in the circuit schematicof FIG. 2, the gain control 37 of thel two-stage amplifier 36 issuitably adjusted such that, for normal operation of the A.C. powersource 10, the negative signal component appearing at the output of thedetector element 38a has a peak value of -4 volts as shown in FIG. 3E.The variable resistance R of the time constant circuit 40 is thenadjusted such that the voltage appearing across the capacitor C, whichis utilized to supply the negative bias potential for the control grid43 of the thyratron tube 45, does not drop appreciably during thatportion of the waveform cycle when the capacitor is discharging. Asillustrated in FIG. 3F, the variable parameter R of the time constantcircuit 40 is adjusted such that the negative voltage appearing acrossthe capacitance C iluctuates between a peak value 130, which issubstantially equal to -4 volts, and a minimum value 132 which is shownas being approximately -3 volts. Thus the RC time constant isillustratively adjusted, as represented in FIG. 4, so that the capacitorC.

only discharges to 75 of its peak value E during that portion of thesignal waveform cycle when the instantaneous voltage derived from thedetector element 38a is less than the instantaneous capacitor voltage e.

The thyratron tube 45 is indicated in FIG. 3F as having a criticalcontrol grid bias level 136 of -2 volts; that is, the thyratron Willfire, for a particular value of plate.k potential, when the control grid43 is at or less negative than -2 volts with respect to the cathode 46.The thy-` ratron draws its positive plate potential from a suitablesource of B+ voltage such as battery 39. Also connected in series in theplate circuit of the thyratron 45 is a solenoid coil 46, which forms apart of the equipment dropout circuit 15, and a normally-closed switch52 which serves to disconnect the plate electrode 44 from the potentialsupply 39 when it is desired to turn oit or reset the thyratron after itis tired. This switch 52 may also be employed to prevent the thyratrontube 45 from being fired when the protective apparatus is initiallyturned on and the capacitor C of the time constant circuit 40 has notyet been suiciently charged up to provide an inhibiting bias level onthe control grid 43 of the thyratron. In some embodiments of theinvention, the supply of B+ potential for the plate circuit of thethyratron tube- 45 may be derived from an element contained in theelectrical system which is to be protected by the proposed apparatus,rather than from a separate supply such as battery 39. In suchsituations, the switch 52 may be eliminated, if desired, as the removalof the electrical equipment 20 from the power line will also effectivelydiscon-y erator and also serve as the plate voltage supply for thethyratron.

As previously explained, during normal, uninterrupted operation of thealternating power source 10, the amplified signal from the photocell34a, which is generated by the pulsing illumination of the glow tube 32ain re- Y sponse to the waveform of the alternating supply, is ofsuilicient magnitude to maintain the control grid 43 of the thyratron 45at a negative bias well above the However, when critical value forfiring of the tube.

The D C. generator` may serve to supply the exciting iield for the A.C.gen-V a power dip or voltage fluctuation such as 70 occurs, themagnitude and duration of the charging voltage pulse supplied to thetime constant circuit 40 by the signal waveform is greatly curtailed, asis represented by the correspondingly small spike of voltage 112 whichappears in the fourth cycle of the rectified signal waveform shown inFIG. 3E during the time interval Ti. As a consequence, the capacitor Cis not fully recharged to its initial peak value E b y t'he shortenedpulse 112 but reaches instead only an intermediate voltage value shownas 134 in FIG. 3F. The capacitor then continues to discharge toward zeropotential during the remainder of the time period T1 when the rectifiedwaveform at the detector output has zero value. As shown in FIG. 3F, thecontrol grid bias potential of the thyratron tube 45 follows thecapacitor discharge until the critical value is reached, as indicated at138, at which point the thyratron fires.

Through variation of the setting of the resistance R in the timeconstant circuit 40, the rate at which the capacitor C discharges,during that portion of the signal cycle when its rectified waveform iseither zero or less than the instantaneous voltage across the capacitor,may be adjusted. Accordingly, tiring or no-liring of the thyratron tube45 by a power dip or voltage fluctuation of predetermined magnitude andduration can be controlled by manipulation of the variable resist-anceR. For the power dip assumed, which lasts for only a portion of a halfcycle of the power supply, the setting of the time constant circuit 40through variation of the resistance R effectively determines whether'ornot the capacitor voltage, and hence the control grid bias voltage, willdrop to the critical firing level of -2 volts during the time intervalT1.

FIG. 5 illustrates the effect produced in the protective apparatusthrough variation of the discharge rate of the capacitor C by adjustmentof the variable resistance element R. Curve 1 shown in FIG. 5 has a timeconstant (RC product) of too short duration, thereby indicating anunsuitable setting for R, as the thyratron 45 would tend to be fired innormal, uninterrupted operation of the A.C. supply 10, since thecapacitor C would discharge down to the critical value of control gridbias in the course of a single cycle of the regular signal waveform.Curves 2 and 3 of this figure exhibit longer time constants, andaccordingly the thyratron tube 45 would not be inadvertently firedduring normal operation of the alternating supply with these settings ofthe resistance R in the time constant circuit 40. Curve 2 shows a timeconstant approximating that shown in FIG. 3F and thus indicates that apower fluctuation lasting as briefly as a fraction of a cycle of thesupply waveform would be sucient to cause the thyratron tube to re.Curve 3 has a somewhat longer time constant than curve 2, and thus thissetting would not result in a firing of the thyratron tube until a powerloss lasting for several cycles of the supply waveform had occurred. Itis to be understood, of course, that variation of the setting of thetime constant circuit 40 could also be effected through suitableadjustment of a variable capacitance element substituted for the fixedcapacitor C.

When the thyratron tube 45 lires, thereby indicating that a powerfluctuation or voltage dip of predetermined magnitude and duration hasoccurred in the waveform of the alternating potential supplied by thepower source 10, a heavy surge of current begins owing in the platecircuit of the tube. This heavy surge of plate current drawn by thethyratron tube at the instant of firing is then utilized as a controlsignal to activate the drop-out circuit which disconnects the electricalequipment 20 from the A.C. power source 10.

An exemplified in FIG. 2, upon the receipt of the current surge orcontrol signal from the plate 44 of the thyratron tube 45, the solenoidcoil 46, which possesses adequate internal resistance to limit themagnitude of the current flow to a suitable value, is sufiicientlyenergized thereby to cause the opening of normally-closed contactor 48.The opening of contactor 48 in turn causes the activation of suitablecircuit-breaking means 50 for rapidly opening the power line and therebydisconnecting the electrical equipment 20 from the A.C. power source 10.As previously stated, the circuit-breaking or circuit-interruptingdevice 15 utilized may be of conventional design whereupon the recipt ofan electrical control signal activates a suitable mechanism fordisconnecting the electrical system which is to be protected from thepower source. The type of drop-out circuitry employed is a matter ofdesign choice and is generally determined by the nature and size of theelectrical system which is to be safeguarded from transient powerfluctuations.

Upon the activation of the circuit breaker 50, the electrical equipment20 is quickly disconnected from the energizing source 10 and the linepreferably remains open, even after the control signal from thethyratron ceases, until the circuit breaker 50 is reset. The electricalequipment 20 is thus protected against the sudden reapplication of fullline potential when the transient fluctuation ends and the supplyvoltage is restored. The protective apparatus of the present inventiontherefore closely monitors the power line waveform and rapidly providesa control signal for activating suitable drop-out circuitry fordisconnecting the electrical system from its A.C. power supply when apotentially harmful condition exists due to the presence of a transientpower fluctuation.

In some applications it may be desirable to utilize the control signalgenerated by the thyratron to perform other functions in addition toactivating an equipment drop-out circuit. One such application is theuse of the control signal from the thyratron as an input to a recorderto monitor and count the number of sudden voltage dips exceeding a givenmagnitude and duration which occur over a given time period.

` Whereas the terms and expressions which have been employed are used asterms of description and not of limitation, and whereas there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described, or portions thereof, itbeing recognized that various modifications are possible within thescope of the teachings of the invention, it is hereby stated and desiredthat the scope of the invention be limited solely by the followingappended claims.

What is claimed is:

1. In a napid-acting system for protecting electrical apparatus againstfluctuations in its alternating electric supply, in combination, afast-response lamp connected to said elect-nic power supply and emittingpulses of light of intensity and frequency related to the amplitude andalterations in said supply, -a lightsensing means including aphotosensitive device positioned to respond to light from said lamp andadapted to generate a pulsating signal related to the waveform thereof,a control device having a lirst state and a second state, meansincluding a waveform-sensing circuit connected to receive said pulsatingsignal from said light-sensing means and also connected to said controldevice and adapted to shift it from said first state to said secondstate in response to a change in'the waveform of said light pulsescaused by a fluctuation in said electric supply, and acircuit-interrupting device connected to `said control device andcontrolled thereby for disconnecting said electrical apparatus from itselectric power supply when said control device is shifted to said secondstate.

2. A combination according to claim 1 in which said lamp is a gas-filledlamp adapted to be illuminated and extinguished at double the frequencyof said 'alternating electric supply, whereby to cause both the lightemitted by said lamp and the pulsating signal generated by said lightsensing means to comprise pulses of double frequency.

3. In a rapid-acting system for protecting electrical apparatus againstiluctuations of a fraction of a cycle or greater in its alternatingelectric power supply, in combination; a fast-response lamp energized bysaid electric power supply and emitting pulses of light having afrequency twice that of said supply; a light-sensing me-ans including aphotosensitive device positioned to respond to light emitted from saidlamp and adapted to generate a puls-ating electrical signal having -awaveform related to the waveform of said emitted light; means includinga detector connected to said light-sensing means, a time constantcircuit connected to said detector, a normally oit discharge tube having-a control electrode connected to said time constant circuit, saiddetector and time constant circuit being responsive to the waveform ofsaid electrical signal from said light-sensing means and adapted to firesaid discharge tube when a power iluctuation of a predeterminedmagnitude and duration occurs in said supply; and `acircuit-interrupting device connected to said discharge tube andcontrolled thereby for disconnecting said electrical apparatus from itselectric power supply when said discharge tube is fired.

4. In a system for protecting electrical apparatus subject to damageresul-ting from extremely short-term transient change of a fraction of acycle or greater in its alternating current supply, in combination, agas-filled lamp connected to said supply and emitting pulses of lighthaving a frequency twice tha-t of said supply, a photosensitive deviceresponsive to illumination from said lamp for generating an electricalsignal of periodic Waveform related to the frequency and intensity ofsaid light pulses, means for amplifying said signal, means including anonlinear device and a time constant circuit for sensing s=aid amplifiedsignal and for generating a bias voltage deter-4 mined by the waveformof said signal, whereby transient changes in said supply produce aresponse in said bias voltage at a rate determined by said time constantcircuit, and rapid-acting circuit-controlling means having an inputterminal connected to an output of said time constant circuit, saidcircuit-controlling means being acti- 12 vated by said -responseproduced in said bias voltage by said transient changes thereby todisconnect said apparatus from its alternating current supply.

5. A combination according to claim 4 wherein said' gas-filled lamp isilluminated when the absolute magnitude of the instantaneous voltage ofsaid alternating current supply exceeds a first, non-zero value, 5andsaid illumina tion is extinguished when the absolute magnitude of thevoltage of said supply falls below a second, non-zero level.

6. A device for detecting a transient fluctuation of a fraction of acycle or greater in an A.C. voltage supply comprising, in combination,fast-response illumination means energized by said A.C. voltage andemitting pulses of light having fa frequency twice that of said supply,a photosensitive element for generating a pulsating electrical signalresponsive to the frequency and intensity of light pulses emitted fromsaid illumination means, means connected to said photosensitive elementfor amplifying said signal, non-linear means for transmitting only aportion of said amplied pulsating signal, energy storage means connectedto said non-linear means for receiving said portion of said signal, anda normally off discharge tube having a control grid which receives abias voltage from said energy storage means, the level of said biasvoltage being determined by the waveform of said portion of said signal,whereby a transient fluctuation in said A.C. voltage of predeterminedmagnitude and duration is reflected in the waveform of said pulsatingsignal and causes said bias voltage to reach `a level causing saiddischarge tube to tire, thereby indicating the occurrence of saidtransient fluctuation.

References Cited by the Examiner UNITED STATES PATENTS 3,004,174 10/61Seidman 328--146 X 3,029,423 4/ 62 Koranye 340-253 SAMUEL BERNSTEIN,Primary Examiner.

1. IN A RAPID-ACTING SYSTEM FOR PROTECTING ELECTRICAL APPARATUS AGAINSTFLUCTUATIONS IN ITS ALTERNATING ELECTRIC SUPPLY, IN COMBINATION, AFAST-RESPONSE LAMP CONNECTED TO SAID ELECTRIC POWER SUPPLY AND EMITTINGPULSES OF LIGHT OF INTENSITY AND FREQUENCY RELATED TO THE AMPLITUDE ANDALTERATIONS IN SAID SUPPLY, A LIGHT-SENSING MEANS INCLUDING APHOTOSENSITIVE DEVICE POSITIONED TO RESPOND TO LIGHT FROM SAID LAMP ANDADAPTED TO GENERATE A PULSATING SIGNAL RELATED TO THE WAVEFORM THEREOF,A CONTROL DEVICE HAVING A FIRST STATE AND A SECOND STATE, MEANSINCLUDING A WAVEFORM-SENSING CIRCUIT CONNECTED TO RECEIVE SAID